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Waist to height ratio.
Your waist divided by your height. It is a better read on where weight is carried than BMI, and the rule is simple enough to remember: keep your waist under half your height.
Why this beats BMI
The sum. Waist divided by height, in the same units. A 34 inch waist on a 5 foot 10 frame is 34 ÷ 70 = 0.49. Under 0.5 is the healthy range, which is where the NHS advice to "keep your waist to less than half your height" comes from.
Why it is a better measure. BMI knows your mass and nothing else, so it cannot tell muscle from fat and cannot tell where fat sits. Waist-to-height looks at central fat specifically, which is the part that carries the health risk, and it needs no equipment beyond a tape measure. If the two disagree, this is usually the more informative one.
The bands. Under 0.4 is below the healthy range and worth a conversation rather than a celebration. 0.4 to 0.5 is healthy. 0.5 to 0.6 is increased risk. Above 0.6 is high. They are population thresholds, not a diagnosis, and they are not adjusted for pregnancy or for children.
Measure it honestly. Tape at the belly button, standing relaxed, breathing out normally, tape snug but not compressing. Do not hold your stomach in and do not use your trouser size, which is a fashion measurement rather than an anatomical one.
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This is an estimate and a starting point, not medical advice. Every equation of this kind is a population average, so your real burn can sit a few hundred kcal either side of it. Track for two or three weeks and adjust to what actually happens on the scales. If you are pregnant, managing a health condition or unsure, speak to a doctor or dietitian first.
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Now log against it
A number is only useful if you can hit it. Whey to Go tracks food and training against targets like these, on your device, with no account and nothing sold.